I'm not exactly satisfied with the number of nice pictures that come from this convention. It used to be located in a huge convention center with good lighting and areas to explore for photo shoots. But since it was in construction, the convention changed placed and.......it wasn't very nice.

There were only one floor, you enter the convention place you see some light hanging from above and concrete floors, that's the hall for rave. There was a room for artists and dealers, then rooms for panels, and that's it. There isn't a less crowded hallway that cosplayers can pose.

In the dark hall, I could have used flash, but I'm never comfortable with the idea of using a flash. I'm still new at it, but as a cosplayer, whenever I'm being flashed at, my make-up look very fake (example here). I'm not sure if that's always the case, please tell me more about it!

I invited a cosplayer to take a few pictures outside, but that didn't turn out to be a great idea. It was hot out, and the sunlight was right above the head, the picture turned out to be blurry (which I don't understand why), and the cosplayer had to squint. Generally, the picture was over exposed, and changing aperture didn't make it much better.

Picture of her inside

Pictures of her outside

I felt very sorry to her.
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A picture of a cosplayer I took in the convention center. The original picture was too dark, and I edited it.

It turned out fine, but I don't see anything special from it, which made me very upset. As a photographer I didn't do much rather than press the shutter and release it, which anyone could have done. The only reason that the picture was nice was because she was a good cosplayer. I tried to pose her, but with a dress like that, I couldn't have her sit on the groud, and the area was just too crouded, we were blocking other people. Now thinking back, I probably should have asked her to take a few more pictures later.
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I think this picture is my favorite picture from convention because of the lighting above their head, though the other cosplayer's face was under the shadow. I could have used a reflector (if I had one), but I kind of like it with her face darkened. The cosplayer probably doesn't like it that way.

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I used the flash for this picture
As you can tell, the flash really made her make-up shown, but not too bad. I really liked her eyes in this picture.
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Another thing that I didn't like a lot about this con is that no one scheduled photo shoot with me prior to the convention. There wasn't any other photographer available for photo shoots either. So all of the pictures I got, I stopped cosplayers passing by and asked for pictures. Worrying that I might have held them back, I didn't really ask them to take off the badges or other belongings they had on themselves. So seeing the things that don't belong in the picture made me upset too.

These two cosplayers were very good, and I hope I had asked them for extra pictures, rather than being one in a crowd taking pictures. Although, I don't feel comfortable to pose them sitting on concrete floor in the middle of crowded hallway ;/

I think your composition is good.
1st - the top has too much empty space.
2nd - need to be more focus on her face.
3rd - excluding the background people, and take that street light off (you can do these on photoshop)

I'm by no means a pro, but hopefully my comments can help out. I'll try to go through what you have one by one.

Using flash in the dark hall - I can't exactly see your makeup in that photo, but I can tell you whoever shot that photo pointed the flash directly at you and didn't diffuse it. You can see how defined and large the shadow casted by the flash is behind you two. Usually you want a soft light so that you can see the details but not blow out anything. You can do that by bouncing flash off the ceiling but at this location, the ceiling is too high so you'll need a bounce card or diffuser. That will soften the light instead of the harsh direct light.

Cosplayer - The 1st photo inside is nice, maybe a little too much empty space in the top portion. Maybe if she had held the big gun on her shoulder, it would have filled it better.
Outside photos - Yeah, you want to avoid harsh sunlight overhead, your subject will always have to squint and there's just too much light. If possible, move them to a shady area, there should be enough natural light.
As for why it's blurry - What aperture/shutter speed were you using?

For the cosplayer in the dress - It's a good photo for showing the entire cosplay, but I don't like how the light is casted on her face. This would have been a good place to use fill flash to light up her face to match the rest of the lighting.
To make the photo more interesting, try different angles. Straight on from the front is usually plain, it gets you a full view of the cosplay but that's about it unless the cosplayer has an awesome pose. But if you're blocking people, there's not much you can do if you're trying to be quick.

Your favorite picture - I pretty much agree with what you thought about it.

The photo you used flash - It's a good shot. Is this the popup flash straight on? Try diffusing it and/or getting a flash that you can angle the lighting.

As for photoshoots - How did you advertise yourself for photoshoots prior to the convention? And if there were no photoshoots at all, maybe the cosplayers weren't expecting to have photoshoots there while the convention was having construction?
At the convention, if you want to shoot more photos of a certain cosplayers, just ask them. Worst that will happen is they say no, and you move on. As for taking off badges, backpacks, etc., again just ask and explain that it will make for a better shot. Unless they are in a rush for a panel or something, most cosplayers I've seen will drop their stuff and strike a pose if asked nicely.

Again, I'm not a pro so maybe I'm off on what I said. One thing that I do though, is compare my shots of cosplayers with other photographers's shots of the same cosplayer and examine the differences. You'll see what works and what doesn't and how you should approach the shot next time. Seeing professional photographers's composition is very helpful.

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Another thing that I didn't like a lot about this con is that no one scheduled photo shoot with me prior to the convention. There wasn't any other photographer available for photo shoots either. So all of the pictures I got, I stopped cosplayers passing by and asked for pictures. Worrying that I might have held them back, I didn't really ask them to take off the badges or other belongings they had on themselves. So seeing the things that don't belong in the picture made me upset too.

These two cosplayers were very good, and I hope I had asked them for extra pictures, rather than being one in a crowd taking pictures. Although, I don't feel comfortable to pose them sitting on concrete floor in the middle of crowded hallway ;/

One thing you might try in the future with this kind of outside shot is to position the cosplayer in the shade right before the area becomes unshaded (direct sunlight). This way the concrete can act as a natural reflector, the cosplayer is not squinting, and there isn't a hard direct light on her and the wall will act as a clean background.

I think if you find cosplayers in the hall and want to get good shots, communication is the key. I ask if they want to take off their badge or I ask them if they don't mind sitting down somewhere or moving to a better location. The worst they can say is "no". I think they want good pictures too, so by asking those questions then you show you care and want to get a good shot. The other option is to stake out place with decent lighting and just out of the way of the major traffic zones. That way you have a good place to shoot and all you have to do is ask a cosplayer walking by if they wouldn't mind taking a few extra steps to get a good shot. You might get fewer shots, but you'll probably be happier with them since you can be more in control. You set take practice shots to figure out what exposure is best for that area so you'll have a better chance of getting the shot you want and shooting faster.